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Go into Curve mode 
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• Move the dot in the curve cursor over a Curve Mark. Place the cursor approximately where you want the new point inserted (any location between the start and end point of the curve but not close to the ends (or that would be an "extend" as described above).
• Click on the right mouse button.
The Curve Right Menu will appear.
Pick "Insert point in curve".
A point will be inserted in the curve at the current mouse position temporarily. Move the mouse until the curve achieves the desired shape.
• Click the left mouse button.
A new curve point will be drawn at the location of the mouse and will be added to the curve. Note that you can click on an existing Marked Point and that Marked Point will be added to the curve. A curve can have Marked Points at any location (not just the two ends).